• International 4000 Series (DuraStar): Fits models 4100, 4200, 4300, and 4400 (years 2002–2018). This is the most common application for hydraulic master cylinders.

• International MV Series: The successor to the DuraStar (2019–present), used for box trucks and towing.

• IC Bus (School & Commercial): Fits CE Series and BE Series buses. If the bus does not have a "hissing" air brake sound, it uses this hydraulic kit.

• International TerraStar: Small-to-medium vocational trucks often found in landscaping or light construction.

• WorkStar (7000 Series): Some lighter configurations of the 7300/7400 models use hydraulic systems instead of air.

• Rubber Cups/Seals: These prevent brake fluid from leaking past the piston and ensure high-pressure buildup.

• Piston Assembly: The metal or plastic rod that moves inside the bore.

• Return Springs: These push the piston back to the starting position when you let go of the pedal.

• Snap Rings & Dust Boots: Hardware to secure the internals and keep dirt out.

Why use a kit? Over time, the rubber seals degrade due to heat and moisture in the brake fluid. A kit restores the cylinder to factory performance, solving issues like a sinking pedal or internal pressure loss, provided the internal bore of the cylinder is not scratched or pitted.

• Sinking Brake Pedal: This is the most classic sign. When you hold the brake pedal down at a stoplight, it slowly but steadily creeps toward the floorboard. This indicates that fluid is leaking past the internal rubber seals (internal bypass).

• Spongy or Mushy Pedal: If the pedal feels soft and you have to pump it several times to get the truck to stop, there is either air in the system or the primary seal in the kit is not creating a tight fit.

• External Leaks at the Booster: Look at the point where the master cylinder bolts to the brake booster (Hydro-Boost). If you see wetness or dripping brake fluid there, the rear seal has failed.

• Brake Dragging: If the return spring inside the kit is broken or the piston is sticking, the brakes may not fully release after you let go of the pedal, causing the brakes to overheat.

• Warning Light (BRAKE or ABS): A faulty master cylinder can cause a pressure imbalance between the front and rear circuits, which trips the pressure differential switch and turns on the dashboard warning light.

• Contaminated Fluid: If you see black streaks or "swirls" in the brake fluid reservoir, the rubber seals inside the cylinder are disintegrating and contaminating the system.